Field of the Invention
This invention relates to a method for producing a highly-expanded non-rigid foamed sheet with cross-linking having a highly homogeneous cell structure for an interior trim material particularly for vehicles, and to a method for producing a light-weight laminated material having soft touch and good cushioning as well as excellent moldability and high-frequency welding workability.
Conventionally, known interior trim materials for vehicles having cushioning properties include laminated materials composed of (1) a surface layer of plasticized polyvinyl chloride sheet and a lower foamed layer of expanded polyolefin resin such as polyethylene, polypropylene, etc., bonded to the surface layer; (2) a surface layer of plasticized polyvinyl chloride sheet and a lower foamed layer of expanded plasticized polyvinyl chloride without cross-linking, united with the surface layer, and (3) a surface layer of plasticized polyvinyl chloride sheet and a lower foamed layer of highly-expanded plasticized polyvinyl chloride with cross-linking laminated to the surface layer.
In regard to the above-described laminated material (1), it is necessary to cross-link foamed resin layers with electron rays, and then to expand them by heating, which results in a large amount of capital investment. Since this method requires complicated treating processes, it is inferior in productivity. In addition, said laminated material is also inferior in high-frequency welding workability because the lower foamed layer is composed of an expanded polyolefin sheet.
In regard to the above-described laminated material (2), the expanded sheet can be produced economically with general production facilities. However, only an extremely low-expanded sheet is obtained, because of its upper expansion coefficient limit of 3-4. Thus, this laminated material lacks, softness, light-weight, and cushioning. In addition, since the foamed layer of said laminated material is non-cross-linking, it is crushed when thermoforming so that molded interior trim material for vehicles can not be obtained.
In regard to the above-described laminated material (3), a soft, light and cushioning product can be obtained because of its highly-foamed layer, but the kneading and dispersing effect of the above-described resin component with a cross linking agent and a blowing agent is imperfect. The crosslinking reaction only proceeds partially, because of insufficiently controlled temperature and treating time during roll-mill kneading and sheeting by a calender, so that it is difficult to homogeneously cross-link and expand it. As a result thereof, an expanded product with homogeneous cross-linking structure and homogeneous cell structure is not obtained, deep drawing moldability in thermoforming is low, and an interior trim material with highly accurate and sharp contour can not be obtained.
In addition, since roll-mills are used for kneading and pelletizing and sheeting is carried out by means of a calender or an extruder, this process is inferior also in productivity.